In hybrid electric vehicles (HEVs) having one or more electric machines or electric traction motor/generators (M/Gs), there is a continuing effort to reduce the noise, vibration, and harshness (NVH), which can be associated with various modes of operation. During operation, M/Gs may generate noise and vibration that may be transmitted to the body and frame of the HEV, causing a perception of harshness by HEV occupants. Attempts have been made to reduce such harshness and the underlying noise and vibration of M/Gs, and include efforts to mitigate torque ripple and related harmonics by structural dampening, and various adjustments to M/G rotor and stator components such as magnets, excitation coil windings, and stator/rotor teeth, which modifies aspects of such noise and vibration.
It is known practice in the design of a brushless, multiple-phase motor-generator to include a stator having stator teeth excited with wire coil windings. The stator commonly defines an interior, cylindrical opening that receives a multi-tooth rotor that includes permanent magnets arranged on its periphery to form the teeth. The stator and rotor teeth interact across a small cylindrical air gap, as magnetic flux pattern develops when the stator coils, wound on the stator teeth, are energized with a multi-phase sinusoidal voltage. The output torque of such a multi-phase M/G includes uniform/fundamental and varying/harmonic torque components. The fundamental component establishes the primary torque output of the M/G. The harmonic and varying torque component creates torque oscillations and vibrations in the torque output, due to harmonic magnetic flux distributions arising from the magnetic response of the air-gap, the stator, and the rotor. The total torque output of the electric machine is the combination of the uniform and varying components, and includes a phenomenon referred to as torque ripple. Torque ripple causes speed oscillations in the HEV drivetrain, which causes vibration and noise as the vibrations resonate in the body and frame of the HEV. Torque ripple occurs at multiple frequencies, but is particularly significant at low vehicle speeds because lower frequency vibration and noise are more difficult to dampen.
The traditional stator tooth shape has a constant width from root to edge, and has a generally rectangular shape. Some have attempted to improve electro-magnetic performance and reduce NVH by incorporating varying widths of such teeth, modifying the air-gap face of the teeth to have an irregular undulating surface, and varying the size of the slots defined between the teeth. Such attempts are directed to achieve an optimal back-EMF shape, among other electromagnetic properties, which reduces torque ripple. Despite these efforts, M/G noise and vibration persist and the need to further reduce the resultant harshness remains.